The National Academies of Sciences, Engineering and Medicine represents the gold standard in the USA for evaluating climate change risks, thus I do not believe that it would be wise for the IPCC bodies working to develop new forcing scenarios to replace the SSP scenarios [see the first attached image from Riahi et al. (2017)] to ignore Steffen et al (2018)'s warnings of the risks of following trajectories leading to a 'hot house' Earth, nor to ignore Scambos et al. (2017); which indicates that: 'Constraining how much and how fast the West Antarctic Ice Sheet (WAIS) will change in the coming decades has recently been identified as the highest priority in Antarctic research (National Academies, 2015)'.
Therefore, in addition to the traditional five families of anthropogenic radiative forcing scenarios that the IPCC normally sponsors for each new Assessment Report; I recommend that for AR7 the IPCC creates a sixth family of forcing scenarios that considers the non-radiative forcing from MICI-type collapses of key marine (and marine terminating) glaciers in both Antarctica and Greenland combined with a postulated major global socio-economic contraction beginning circa 2050, where it would be assumed that the world would abruptly transition from its current BAU (or SSP 5) pathway to an essentially carbon neutral pathway due to the postulated contraction circa 2050. Fundamentally, I am recommending that the IPCC follow the approach used by Hansen et al. (2016), but to develop a family of more rigorous scenarios based on ice sheet forcing risks starting with the CMIP6 projections that consider MISI-type ice-climate feedbacks (which likely contributed to many of these models projection values of ECS equal to, or greater than, 5C).
Due to the complexities of MICI-type of mechanism, I would further recommend that the IPCC use the output from the eight different projects of the 5-year (starting in 2018) International Thwaites Glacier Collaboration (see the second image) in 2023-2024 to create regional models of both Antarctica and Greenland in order to develop values for the non-radiative forcing from MICI-types of collapses from key marine (& marine terminating) glaciers; to create this sixth IPCC sponsored family of non-radiative forcing scenarios (starting circa 2040 to 2050) that would be input into CMIP7 models to provide input into AR7.
Examples of variables that scientists (possibly advised by such experts as: Rignot, Hansen, Alley, Pollard and DeConto) could use to develop different individual scenarios for such a sixth family of both radiative and non-radiative forcing scenarios could/might include:
1. Key governments (such as the USA) implementing aerosol-based geoengineering between 2040 and 2050; that cannot be sustained after 2050 due to the postulated global socio-economic contraction.
2. Key marine terminating glaciers (such as Jakobshavn) providing a multi-year armada of icebergs into the North Atlantic (say between 2030 & 2035) that might trigger a collapse of key Antarctic Peninsula ice shelves (say Larsen C) due to the bipolar seesaw mechanism; that might trigger an increase in upwelling of warm CDW across key Antarctic continental shelves that might increase the risks of local MICI-type failures of local marine glaciers (like Thwaites).
3. An extreme El Nino event circa 2030 could drive a seasons worth of warm CDW into the ASE in 2031 that could hollow-out the Big Ear subglacial cavity in the Thwaites gateway by 2033 that could lead to a collapse of that subglacial cavity leading to a surge of the glacial ice feeding the Thwaites Ice Tongue causing it to collapse and to an outburst of the subglacial meltwater from the Thwaites subglacial drainage system (including the previously mentioned four subglacial lakes) that could push out to sea any mélange of icebergs seaward of the ice cliff in the Thwaites gateway.
I could cite other variables cited in this thread but hopefully if the IPCC were wise enough to appoint scientists like Rignot, Hansen, Alley, Pollard and/or DeConto to the team developing such non-radiative forcing contributions to the sixth suite of forcing scenarios, they would have no trouble developing appropriate combinations of variables for different scenarios (hopefully with input from the researchers from at least the International Thwaites Glacier Collaboration):
Will Steffen, Johan Rockström, Katherine Richardson, Timothy M. Lenton, Carl Folke, Diana Liverman, Colin P. Summerhayes, Anthony D. Barnosky, Sarah E. Cornell, Michel Crucifix, Jonathan F. Donges, Ingo Fetzer, Steven J. Lade, Marten Scheffer, Ricarda Winkelmann, and Hans Joachim Schellnhuber (August 14, 2018),"Trajectories of the Earth System in the Anthropocene", PNAS, 115 (33), 8252-8259,
https://doi.org/10.1073/pnas.1810141115 https://www.pnas.org/content/115/33/8252Abstract: "We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a “Hothouse Earth” pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene. We examine the evidence that such a threshold might exist and where it might be. If the threshold is crossed, the resulting trajectory would likely cause serious disruptions to ecosystems, society, and economies. Collective human action is required to steer the Earth System away from a potential threshold and stabilize it in a habitable interglacial-like state. Such action entails stewardship of the entire Earth System—biosphere, climate, and societies—and could include decarbonization of the global economy, enhancement of biosphere carbon sinks, behavioral changes, technological innovations, new governance arrangements, and transformed social values."
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T.A. Scambos et al. (2017), "How much, how fast?: A science review and outlook for research on the instability of Antarctica's Thwaites Glacier in the 21st century", Global and Planetary Change, Volume 153, June 2017, Pages 16-34
https://www.sciencedirect.com/science/article/pii/S092181811630491XAbstract: "Constraining how much and how fast the West Antarctic Ice Sheet (WAIS) will change in the coming decades has recently been identified as the highest priority in Antarctic research (National Academies, 2015). Here we review recent research on WAIS and outline further scientific objectives for the area now identified as the most likely to undergo near-term significant change: Thwaites Glacier and the adjacent Amundsen Sea. Multiple lines of evidence point to an ongoing rapid loss of ice in this region in response to changing atmospheric and oceanic conditions. Models of the ice sheet's dynamic behavior indicate a potential for greatly accelerated ice loss as ocean-driven melting at the Thwaites Glacier grounding zone and nearby areas leads to thinning, faster flow, and retreat. A complete retreat of the Thwaites Glacier basin would raise global sea level by more than three meters by entraining ice from adjacent catchments. This scenario could occur over the next few centuries, and faster ice loss could occur through processes omitted from most ice flow models such as hydrofracture and ice cliff failure, which have been observed in recent rapid ice retreats elsewhere. Increased basal melt at the grounding zone and increased potential for hydrofracture due to enhanced surface melt could initiate a more rapid collapse of Thwaites Glacier within the next few decades."
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Keywan Riahi et al. (January 2017), "The Shared Socioeconomic Pathways and their energy, land use, and greenhouse gas emissions implications: An overview", Global Environmental Change, Volume 42, Pages 153-168,
https://doi.org/10.1016/j.gloenvcha.2016.05.009https://www.sciencedirect.com/science/article/pii/S0959378016300681Extract: "While the SSPs were designed to represent different mitigation and adaptation challenges, the resulting narratives and quantifications span a wide range of different futures broadly representative of the current literature. This allows their subsequent use and development in new assessments and research projects. Critical next steps for the community scenario process will, among others, involve regional and sectoral extensions, further elaboration of the adaptation and impacts dimension, as well as employing the SSP scenarios with the new generation of earth system models as part of the 6th climate model intercomparison project (CMIP6)."
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Title: "The International Thwaites Glacier Collaboration"
https://thwaitesglacier.org/&
https://thwaitesglacier.org/projectsExtract: "UK and US scientists are collaborating to investigate one of the most unstable glaciers in Antarctica, the Thwaites Glacier, roughly the same size as Florida or Britain."